1. Field of the Invention
The present invention relates generally to an acceleration sensor for use in an air bag device carried by an automobile, and more particularly, to an acceleration sensor having a fault diagnostic device.
2. Description of the Prior Art
In an air bag device carried by an automobile, the air bag device operated in response to acceleration applied at the time of, for example, a collision. In order to ensure the operation of the air bag device, an acceleration sensor has been conventionally incorporated in the above described air bag device. As this type of acceleration sensor, an acceleration sensor using a piezoelectric element which is deformed in response to acceleration applied thereto to output an electric signal has been proposed, as disclosed in, for example, U.S. Pat. No. 4,700,973.
One example of a conventionally known acceleration sensor utilizing a piezoelectric element will be described with reference to FIGS. 1 to 3.
FIG. 1 is a schematic block diagram for explaining the construction of a conventional acceleration sensor. There is provided a piezoelectric element 1 outputting, when acceleration G is applied, an electric signal corresponding to the acceleration G. An impedance converter means 2 is electrically connected to the piezoelectric element 1. The impedance converter 2 converts the impedance of the electric signal applied from the piezoelectric element 1. A filter 3 having a band-pass filter is electrically connected to the impedance converter 2. In the filter 3, an unnecessary signal, that is, an out-of-band signal component is attenuated. An amplifier 4 is electrically connected to the filter 3. In the amplifier 4, an output signal applied from the filter 3 is amplified. This acceleration sensor has the piezoelectric element 1, the impedance converter 2, the filter 3 and the amplifier 4, and output a voltage signal corresponding to the acceleration G from an output terminal B.
The voltage signal outputted from the output terminal B of the acceleration sensor is applied to a control unit 5 comprising a microcomputer arranged outside the acceleration sensor. The control unit 5 causes an air bag device for an automobile (not shown) to perform a necessary operation on the basis of the voltage signal applied.
FIGS. 2 and 3 are respectively a plan sectional view showing the specific construction of the above described acceleration sensor and a cross sectional view taken along a line 3--3 shown in FIG. 2. In an acceleration sensor 6, a base plate and a cap 8 secured to the upper surface of the base plate constitute a package structure containing a housing space. Within the package structure, a hybrid IC 9 is secured on the base plate 7 using adhesives (not shown). The hybrid IC 9 is used for constructing the impedance converter 2, the filter 3 and the amplifier 4 described above. A plurality of electrodes 9a to 9f for connection to outer portions are formed on the upper surface of the hybrid IC 9. Each of the electrodes 9a to 9d is electrically connected to a lead terminal 11 by a lead wire 10. A plurality of lead terminals 11 are passed through the base plate 7 and are extended downward.
On other hand, a supporting base 12 is secured on the base plate 7 using adhesives (not shown) beside the hybrid IC 9. A piezoelectric element 1 having electrodes (not shown) on both its major surfaces is secured on the supporting base 12 in a cantilevered shape. The electrode on the upper surface of the piezoelectric element 1 is electrically connected to the electrode 9f by a lead wire 10f, and the electrode on the lower surface of the piezoelectric element 1 is electrically connected to the supporting base 12. In addition, the supporting base 12 is electrically connected to the electrode 9e on the hybrid IC 9 by a lead wire 10e.
The above described piezoelectric element 1 has a series type bimorph structure which is low in piezoelectric voltage.
Meanwhile, the above described package structure constituted by the base plate 7 and the cap 8 is hermetically sealed, and an inert gas, for example, nitrogen gas is sealed into the package structure so as to prevent oxidation. When the above described acceleration sensor is incorporated in the air bag device for an automobile, the air bag device is operated in response to an output signal of the acceleration sensor. Accordingly, the acceleration sensor requires very high reliability, and a fault in the acceleration sensor must be quickly detected. Since a fault diagnostic function has not been conventionally added to the acceleration sensor itself, however, the fault diagnosis of the acceleration sensor is generally made by an outer fault diagnostic device which is provided separately from the acceleration sensor. When there occurs a fault such as the cracking of the piezoelectric element 1 in the acceleration sensor or the stripping of the piezoelectric element 1 from the supporting base 12, the fault may not, in some cases, be quickly found out because the acceleration sensor does not have a fault self-diagnostic function.
Furthermore, the conventional acceleration sensor 6 shown in FIGS. 2 and 3 also has the following disadvantages. That is, when an excessive shock is externally given, there occurs a state where the supporting base 12 fixed to the base plate 7 using adhesives is stripped from the base plate 7 and the piezoelectric element 1, along with the supporting base 12, is not fixed to the other members, as represented by an imaginary line in FIG. 3. Accordingly, acceleration cannot, in some cases, be accurately detected by the piezoelectric element 1. Even if such a fault occurs, the electrical connection between the piezoelectric element 1 and the hybrid IC 9 is ensured through the lead wires 10e and 10f so long as the piezoelectric element 1 is fixed to the supporting base 12, so that the occurrence of the fault may not, in some cases, be detected.